Add automated test for "Enable and disable user account"

The manual test is here: Enable and disable user account

Existing automated coverage

• UserProfileIT#toggleEnableDisable: As admin clicks Disable this account / Enable this account on the profile and asserts the buttons switch and persist after reload.
• UserProfileIT#disabledUserTest: Disables the user from the profile and asserts the disabled user can't save a page over REST (401).

Missing automated coverage

• Disable a user from its profile, log in as that user through the UI
• Open an application from the Applications panel, then Page Index, User Index and Application Index from the drawer, and assert each shows the notice 'Your account has been disabled. Please contact the administrator if you think this is a mistake.' (key accountdisabled, template userinactive.vm) instead of the content
• As admin enable the account again, log in as the user and assert the same pages are displayed and a page the user may edit can be edited and saved

Suggested location: xwiki-platform-core/xwiki-platform-user/xwiki-platform-user-profile/xwiki-platform-user-profile-test/xwiki-platform-user-profile-test-docker/src/test/it/org/xwiki/user/test/ui/UserProfileIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).